1. Field of the Invention
The present disclosure relates generally to the use of neutron emission to detect a substance of interest in a remote target via gamma ray fluorescence, and more specifically toward a method and system for improved gamma ray signal detection.
2. Related Art
The use of emitted neutrons to identify substances of interest in distant targets is an emerging technology. For example, the apparatus and methods described in U.S. Pat. No. 7,573,044, issued Aug. 11, 2009 to Norris, the inventor of the subject invention, directs a neutron beam of thermal, epithermal, or cold neutrons toward a remote target or area to interrogate for possible substances of interest. It is to be understood that the term “target” may be a specific suspicious object like a suitcase or a parked car, an indiscriminate area under investigation, or almost any other desired focus of an interrogation. The detection and analysis of gamma rays returning from the target enable a determination whether the target or its immediate surroundings contain significant concentrations of the substance(s) of interest. Substances of interest may be hostile, as in hidden explosives detection, or benign, as in the detection of certain minerals in connection with mining operations. Diverse applications of this technology abound. The entire disclosure of U.S. Pat. No. 7,573,044 is hereby incorporated by reference and relied upon.
Neutrons sent to interrogate a target will typically produce a broad range of reactions both in the target and also in virtually all other substances that are exposed to the neutrons including, but not limited to, materials in the neutron source itself, shielding and the materials used to produce the neutrons, the intervening atmosphere with all its constituents, materials surrounding the target in all directions (including objects beside, in front of, and behind the target), and uninteresting substances commingled with the target.
Signals resulting from interactions with the target are referred to as signals of interest. Signals resulting from interactions other than from the target are referred to variously as background signals, nuisance signals, or simply as noise. Gamma ray signals received by the detector are the sum of all signals produced by all sources, including signals of interest and noise. The challenge for all detection systems is to distinguish between signals of interest and signals due to noise.
Two significant contributors to noise gamma rays in the field of neutron fluorescence are fratricidal and backshine gammas. All neutron sources tend to directly produce unwanted gamma rays as byproducts. These byproduct gamma rays are sometimes referred to as “fratricidal” gamma rays, since they typically degrade the mission of the device from which they emanate. Gamma ray detectors used as part of a substance identification system cannot be completely shielded from fratricidal gamma rays. Gamma rays produced as a direct result of neutron production thus constitute a source of noise or nuisance signal for the gamma ray detectors. Further, unwanted or nuisance gamma rays may be produced by the interaction of the neutron beam with atmospheric nitrogen as the beam travels toward the substance of interest. These unwanted gamma rays are sometimes referred to as “atmospheric backshine gammas” or “atmospheric sparkle gamma rays,” and constitute additional noise or nuisance signals. The ratio of signals of interest to signals due to noise is known as the “Signal-to-Noise Ratio” (SNR).
One proposed strategy for reducing signal noise is to implement methods that reduce the production of unwanted gamma signals. For example, the applicant's co-pending patent application Ser. No. PCT/US09/65706, filed Nov. 24, 2009, describes strategies for modulating the illuminating neutron beam flux to improve the SNR, among other objectives, by adjusting the signal of interest. Background noise effects are reduced relative to the total received signal using target-distance measurement, other parameters, and computerized algorithms to modulate the neutron beam flux. The entire disclosure of patent application Ser. No. PCT/US09/65706 is hereby incorporated by reference and relied upon.
Other prior art examples include U.S. Pat. No. 7,430,479 to Holslin et al., issued Sep. 30, 2008, which discloses a method for interrogating suspicious objects for hidden explosives or contraband via gamma ray emission stimulated by a pulsed neutron source. A “fast” neutron interrogation beam is used (14 MeV) as opposed to a “thermal” neutron interrogation beam. A gated gamma ray detector monitors the return of gamma rays between pulses, with the die-off of the slow-decay species being observed over time for the purpose of substance analysis. The pulse provides quiet “gates” for resolving or disambiguating neutron burst/inelastic gammas, capture gammas, and activation gammas.
U.S. Publication No. 2008/0203309 to Frach et al., published Aug. 28, 2008, describes a gamma ray detector system with a time of flight positron emission tomography imaging feature. In particular, a time of flight processor localizes a positron-electron annihilation event along the line of response based on a time difference between two substantially simultaneous gamma ray detections. This publication recognizes a “time of flight” event with respect to emitted neutrons.
U.S. Pat. No. 5,153,439 to Gozani et al., issued Oct. 6, 1992, describes a fairly basic application of thermal neutron activation analysis techniques, where the use of neutrons in general, with unspecified energy, is used with an Artificial Neural Network to process gamma ray data. The description includes a discussion of subtracting noise from a returning gamma ray signal, but without teaching a specific method for recognizing and discriminating noise from good signal.
U.S. Pat. No. 5,838,759 to Armistead, issued Nov. 17, 1998, describes an inspection system for cargo containers using a fast neutron interrogation beam. Neutrons penetrating the container are thermalized or slowed by several ambient mechanisms, including inelastic and elastic scattering, but the degree of thermalization is not and cannot be controlled, since the neutrons encounter unknown quantities and geometries of numerous atomic species. Such neutrons as are thermalized by this process are then absorbed by certain elements in the target which give off gamma rays for detection.
U.S. Patent Publication No. 2010/0025573 to Hahto et al., published Feb. 4, 2010, proposes a method to produce short neutron pulses at a current of more than 1 milliamp of protons at 9.17 MeV.
Challengingly low SNR is a common problem encountered generally in this field. High levels of noise or nuisance gamma rays make the detection of gamma rays of interest more difficult. Despite the abundance of development activity in this area, there remains a desire for improved signal detection methods to provide faster and more accurate identification of substances of interest in a remote target. Specifically, there exists a need to address the issue of unwanted fratricidal-type and atmospheric backshine type gamma rays in connection with detection strategies in this field.